This invention relates to a control system for a vehicle safety device.
A system for controlling a vehicle safety device, such as an air bag, which utilizes a microcomputer, is already known, for example, from Japanese Laid-Open (Kokai) Utility Model Application No. 5371/90. The microcomputer integrates a deceleration signal from an acceleration sensing circuit, and compares this integral value with a threshold level to determine whether or not a vehicle collision has occurred. The microcomputer, when judging that the vehicle collision has occurred, feeds an ON-instruction signal to a transistor of a drive circuit to turn on this transistor, so that a squib connected in series with this transistor is supplied with electric current and is ignited, thereby expanding the air bag so as to ensure the safety of the passenger of the vehicle. Recently, this control system has come into the limelight because of the high-precision collision judgment effected by the microcomputer.
In the above control system, it is necessary to give consideration to a run-away of the microcomputer. When the microcomputer runs away, it is possible that the ON-instruction signal may be outputted despite the fact that a collision has not actually occurred, with the result that the air bag is activated inadvertently.
In the above control system, a safety switch is used as one of means for dealing with the run-away of the microcomputer. This safety switch is turned on by an inertia force produced upon collision of the vehicle, and is connected in series with the above transistor. When an actual collision does not occur, the safety switch is in the OFF state, and therefore even when the transistor is turned on due to a runaway of the microcomputer, electric current is not supplied to the squib, thus preventing an inadvertent activation of the air bag.
In the above control system, when the acceleration sensing circuit is subjected to a malfunction, the ON-instruction signal is not fed from the microcomputer to the transistor. Therefore, when a collision occurs during the malfunction of the acceleration sensing circuit, the transistor remains in the OFF state, though the safety switch is turned on. As a result, the air bag is not activated.
Japanese Laid-Open Patent Application No. 241467/88 (corresponding to U.S. Pat. No. 4,950,914) discloses means for detecting a malfunction of an acceleration sensing circuit. One of the inventors of the present invention is a joint invention of a U.S. patent application filed on Jan. 18, 1991 which is directed to a control system of the type in which a malfunction of an acceleration sensing circuit is detected by a microcomputer.